Usually it means to keep somthing quiet and to only tell people who are cool enough to know or trustworthy. This definition comes from the real meaning which is a black man/men who have sexual relations with other men but have relationships with women and hide the homosexuality and homo. sex.

yo man i gota tell ya somthin but u gota keep it on the Down Low

u see that guy over there...well hes one of those guys in that Down Low thing, they call it poker night